Value documents according to the invention are bank notes, check forms, shares, certificates, postage stamps, air tickets, labels for product protection and the like. The simplifying designation “security paper” or “value document” will hereinafter therefore always include documents of the stated type.
Bank notes are normally manufactured from so-called security papers consisting of cotton fibers and having special security features, such as a security thread at least partly incorporated into the paper, and a watermark. The period of circulation of a bank note depends on how much it is stressed. Certain denominations are preferably used in trade and thus have a shorter period of circulation due to the stronger load through environmental influences. The main cause for a restricted period of circulation of bank notes is seen to be early soiling. Since bank note paper is very porous, it has a large surface area or high surface roughness. Even if the resulting projections and cavities are in orders of magnitude that cannot be resolved by the human eye, they offer ideal conditions for dirt deposits in comparison with a smooth surface.
It has therefore been suggested in WO 98/15418 to manufacture bank notes completely from a plastic substrate. However, in this case the usual and time-tested security elements such as portrait watermark and windowed security thread must be forgone, as well as the special properties such as sound and feel of bank note paper. Also, the steel gravure printing usual in the bank note sector, which serves as an additional tactile authenticity mark due to the relief arising from the inking, leads only to a flat, hardly noticeable relief on plastic substrates. Moreover, pure plastic bank notes are very sensitive to heat, so that plastic substrates have a strong tendency to shrink particularly in countries with unfavorable climatic conditions. In particular with regard to falsification security, plastic bank notes have the decisive disadvantage that the substrate is readily available and can be procured by anyone.
WO 96/28610 has suggested lacquering, i.e. coating, paper bank notes as an alternative. This does reduce the soiling problem but cannot increase the mechanical stability of a substrate.
Multilayer substrates comprising a film core coated on both sides with paper plies have also been proposed. The strength of this laminate is increased in comparison with a pure paper substrate, but the soiling problem with the exterior paper layers is not solved. So as not to excessively increase the total thickness of such bank notes, the paper plies must moreover be designed very thin, so that the usual security features such as security thread or watermark can no longer be optimally incorporated.